To evaluate the expression \( \frac{b + 15}{b} \) for \( b = -3 \), we first substitute \( -3 \) for \( b \):
\[ \frac{-3 + 15}{-3} \]
Now, simplify the expression in the numerator:
\[ -3 + 15 = 12 \]
So the expression becomes:
\[ \frac{12}{-3} \]
Now, perform the division:
\[ \frac{12}{-3} = -4 \]
Thus, the simplified answer is:
\[ \boxed{-4} \]